Dne 23.1.2019 v 13:46 Leo Yan napsal(a): > Hi all, > > On Wed, Jan 23, 2019 at 12:58:51PM +0100, Takashi Iwai wrote: >> On Tue, 22 Jan 2019 21:25:35 +0100, >> Mark Brown wrote: >>> >>> On Mon, Jan 21, 2019 at 03:15:43PM +0100, Jaroslav Kysela wrote: >>>> Dne 21.1.2019 v 13:40 Mark Brown napsal(a): >>> >>>>> It was the bit about adding more extended permission control that I was >>>>> worried about there, not the initial O_APPEND bit. Indeed the O_APPEND >>>>> bit sounds like it might also work from the base buffer sharing point of >>>>> view, I have to confess I'd not heard of that feature before (it didn't >>>>> come up in the discussion when Eric raised this in Prague). >>> >>>> With permissions, I meant to make possible to restrict the file >>>> descriptor operations (ioctls) for the depending task (like access to >>>> the DMA buffer, synchronize it for the non-coherent platforms and maybe >>>> read/write the actual position, delay etc.). It should be relatively >>>> easy to implement using the snd_pcm_file structure. >>> >>> Right, that's what I understood you to mean. If you want to have a >>> policy saying "it's OK to export a PCM file descriptor if it's only got >>> permissions X and Y" the security module is going to need to know about >>> the mechanism for setting those permissions. With dma_buf that's all a >>> bit easier as there's less new stuff, though I've no real idea how much >>> of a big deal that actually is. >> >> There are many ways to implement such a thing, yeah. If we'd need an >> implementation that is done solely in the sound driver layer, I can >> imagine to introduce either a new ioctl or an open flag (like O_EXCL) >> to specify the restricted sharing. That is, a kind of master / slave >> model where only the master is allowed to manipulate the stream while >> the slave can mmap, read/write and get status. > > In order to support EXCLUSIVE mode, it is necessary to convert the > /dev/snd/ descriptor to an anon_inode:dmabuffer file descriptor. > SELinux allows that file descriptor to be passed to the client. It can > also be used by the AAudioService. Okay, so this is probably the only point which we should resolve for the already available DMA buffer sharing in ALSA (the O_APPEND flag). I had another glance to your dma-buf implementation and I see many things which might cause problems: - allow to call dma-buf ioctls only when the audio device is in specific state (stream is not running) - as Takashi mentioned, if we return another file-descriptor (dma-buf export) to the user space and the server closes the main pcm file-descriptor (the client does not) - the result will be a crash (dma buffer will be freed, but referenced through the dma-buf interface) - the attach function calls dma_buf_get(fd), but what if fd points to another dma-buf allocation from a different driver? the unexpected private data will cause crash - there should be a type checking in the dma-buf interface If I look to the dma_buf_fd() implementation: fd = get_unused_fd_flags(flags); fd_install(fd, dmabuf->file); .. what if we just add one new ioctl to the ALSA's PCM API which will return a new anonymous inode descriptor with the restricted access to the main PCM device to satisfy the SELinux requirements / security policies? It might be more nice and simple solution than to implement the full dma-buf interface for the ALSA's PCM devices. Question: The dma-buf also implements the fencing, but I am not able to determine, if this mechanism is used in android [1]. It may allow concurrent mmap and synchronize apps - but the sound server should manage the access to the DMA buffer anyway. In my opinion, it makes much sense for the video-pipes when the hardware does some accelerations (encoding/decoding). Jaroslav > [1] https://source.android.com/devices/audio/aaudio -- Jaroslav Kysela <perex@xxxxxxxx> Linux Sound Maintainer; ALSA Project; Red Hat, Inc. _______________________________________________ Alsa-devel mailing list Alsa-devel@xxxxxxxxxxxxxxxx http://mailman.alsa-project.org/mailman/listinfo/alsa-devel
